Honda made only 14,000 gen-1 Insights, and the aftermarket (eg. Bumblebee Batteries) for that car arose to keep these cars on the road (I performed my own battery swap with guidance from YouTube). Aftermarket support for the Clarity will be more likely because many more Claritys were produced and because the Clarity was more valuable than a Spark to begin with.
True, the gen-1 Insight had 144 NiMH D-cell batteries, which was state-of-the-art for 1999. Although the Clarity's Li-Ion battery is more complicated, I believe the battery industry has grown so much since 1999 that there will be alternative component batteries that technicians can use. The key factors will be the size of the market and the value vs investment calculation. If someone wants a luxuious plug-in hybrid car, the Clarity and the rare Cadillac ELR stand alone (unless you consider the Volt luxurious). So the market for used Claritys may continue, but only if the cost of an aftermarket battery is reasonable.
There was a member here, @Cash Traylor who purchased a complete Clarity battery on eBay. It was delivered to his home and he proceeded to completely dismantle it. His objective was to re-deploy the modules for a variety of purposes ranging from home energy off-grid to a go-cart for his daughter. There was a time when he was pretty active here, but I haven't heard a peep from him in more than a year. I doubt he is still listening because his user name doesn't populate correctly with the '@' any more.
Another unknown situation that he might have had some insight into is the DC-DC converter, which is built into the battery assembly. Just anecdotally someone who looked at a Clarity battery said that it appears it would be very difficult to replace the DC-DC converter as it is buried inside all of the other electronics. Don't know how true that is but in at least one case someone who had their DC-DC converter fail under warranty the dealer swapped out the entire battery. So that may be the case post-warranty also. I have read, again anecdotally that it's relatively easy to remove and re-install a Clarity battery, so there will likely be independent repair shops available where people can go to get a new or refurb Clarity battery installed. However those same repair shops may not have the expertise to replace the DC-DC converter within the battery if that's a really specialized procedure, the result being that a failed DC-DC converter will require getting a new or refurb battery. Which might be the better thing to do anyway, assuming that the battery is already old at that point, and I'm guessing the parts and labor cost of replacing just the DC-DC converter would be pretty expensive if done separately, and again it may be difficult to find a place that can even do it, maybe not even at a dealer. So if that's all true, then realistically the DC-DC converter would best be thought of as just one of the parts of the battery, meaning that just like any other part of the battery if it fails then a new or refurb battery will be needed.
My local Honda dealer had to buy an expensive hydraulic battery lift to service Clarity batteries. Any shop that services BEVs will likely have such a lift, however shops that service BEVs are still few and probably far between in most parts of the country.
I'm sure that's true, I didn't mean to imply that most service shops can replace a Clarity battery, I meant the type of shops that have experience with hybrids and EV's. But yes living in a region where EV's are rare will likely make that quite a bit harder to find. And I'm still surmising that even shops that work on EV's probably won't replace battery cells (or DC-DC converters) and will instead just install complete new or refurbed batteries. Kind of like the old days when you needed your carburetor rebuilt, very few shops did that in house, instead they would install a rebuilt carburetor that came from a place that specialized in doing that.